This invention relates to a high-accuracy wheel spacer used in an off-vehicle wheel servicing machine, such as an electronic spin balancer. The balancer incorporates a mid-centering device comprising a horizontal mounting shaft extending outwardly from an electric motor housing, and an annular “face plate” fixed to the mounting shaft near the housing. The vehicle wheel is applied to the mounting shaft through its center hole, and is urged against the face plate using known suitable clamping means, such as a wingnut tightener. The spacer of the present invention is used when servicing large wheels, such as those provided on sport utility vehicles and commercial trucks. Unlike smaller wheels found on most passenger vehicles, larger wheels have a relatively deep rim which often hits against the motor housing and prevents proper mounting of the wheel against the face plate. The present spacer sits directly against the face plate of the mid-centering device, and serves to space the vehicle wheel a clearing distance from the motor housing.
Proper wheel balancing promotes a smooth vehicle ride and helps maximize the life of the tires. An out-of-balance wheel produces an annoying vibration or “shake” which intensifies in direct proportion to vehicle speed. The up-and-down shaking of the wheel is hard on the suspension, adversely effects vehicle handling, and can cause cupping and premature wear of the tire tread.
Almost all service facilities and tire stores today use an off-vehicle electronic spin balancer to balance vehicle wheels. The tire and wheel are applied to a horizontal mounting shaft, urged tightly against the face plate of the balancer, and then spun to find any heavy spots on the wheel. The balancer then indicates where weights (and how much weight) need to be placed to counterbalance the heavy spots. Off-vehicle spin balancers actually check two kinds of balance—“static” and “dynamic.” Static imbalance causes a wheel to shake up and down as it spins, so static balance is achieved when both halves of the tire wheel assembly weigh exactly the same. Dynamic imbalance causes a tire and wheel to wobble back and forth, or sideways, as it spins. Dynamic balance is achieved when the front and back sides of the wheel and tire weigh the same. To achieve proper balancing, the wheel must be clamped in a precisely centered condition on the balancer mounting shaft, both statically and dynamically. As a general rule of thumb, the best balance on an average size tire and wheel assembly is achieved when residual static imbalance is less than ¼ ounce, and residual dynamic imbalance is less than ¼ ounce per plane. In order to attain this precision, the wheel balancer has highly accurate surfaces which must remain clean and substantially wear-free.
After sliding the wheel onto the balancer mounting shaft, the wheel is generally forced directly against the face plate of the balancer and clamped into position using standard components, such as a common flange plate and threaded wingnut tightener. The face plate is formed of steel which is precisely accurately machined and often ground. For larger wheels, a spacer is required to provide sufficient clearance between the wheel rim and motor housing of the balancer, as described above. Prior art wheel spacers typically contact the entire surface area of the balancer's face plate. Since the face plate generally collects a small amount of dirt, dust, and other minute debris during normal shop use, the spacer is often forced by this debris to sit very slightly “titled” against the face plate, thereby creating a dynamic imbalance in the wheel. Thus, because of deficiencies and limitations of prior art spacers, the expensive and precise construction of the balancer face plate is generally negated.
The present invention addresses the need for a highly accurate spacer which effectively locates the wheel a spaced distance from the balancer motor housing. The invention cooperates with clamping elements to tightly mount the vehicle wheel on the balancer shaft in a precisely accurate dynamically centered condition notwithstanding the presence of dirt or other minute debris on the face plate.